I'm replying my question. I don't know if there is any elaborate way but I just removed Helvetica and made a symlink to the font I want. That changed the system fonts. Hope this helps anyone who wants to change the fonts.

I'm replying my question. I don't know if there is any elaborate way but I just removed Helvetica and made a symlink to the font I want. That changed the system fonts. Hope this helps anyone who wants to change the fonts.

Cool way to handle that. But did you find out how you can change the default size of the system font? I have found out that the optimal font size for me seems to be something like 16pt which unfortunately is not availble as Helvetica.